An integrated circuit (IC) may be tested at various stages during the production and use of the IC. For example, tests may be performed following production to verify that the IC exhibits functional and parametric characteristics that conform to the specification for the IC. IC testing typically involves programmable automatic test equipment (ATE) that includes a plurality of signal generation channels for generating test signals that are applied to the input and output (IO) pins of the IC. The ATE evaluates the response of the IC to the test signals.
Typical tests performed on a digital IC include timing tests such as determining the setup and hold time of a data signal with respect to a clock signal. Timing tests may be performed by programming an ATE to vary the delay between a data signal and a clock signal. The delay values that cause the IC to operate correctly are determined and compared to the specifications for the IC.
Tests such as the described timing tests may be adversely affected by internal features of the IC. For example, an IC may include an internal skew corrector (or variable phase shifter) that adjusts the delay of one signal, e.g. a clock signal, with respect to another signal, e.g. a data signal. An example of this type of variable phase shifter is described in U.S. Pat. No. 5,066,868 entitled APPARATUS FOR GENERATING PHASE SHIFTED CLOCK SIGNALS that issued to J. H. Doty, II et al. on 19 Nov. 1991. An internal skew corrector such as that disclosed by Doty, II et al. includes analog circuitry. The timing characteristics of analog circuitry may vary significantly as IC fabrication process parameters vary causing the timing of the skew corrector to be different in each IC. It might be necessary, therefore, to adapt the timing of an ATE test program to each IC before actually performing timing tests. Adapting the test program to the characteristics of each chip would lengthen the time required for testing each IC. Increasing the duration of testing is undesirable, particularly during production when large numbers of integrated circuits must be tested.
To facilitate testing, therefore, it may be desirable to bypass an internal feature, such as a skew corrector, during testing. For example, a skew corrector may be bypassed by including a two-to-one multiplexer (2:1 MUX) in the IC. One input of the MUX is coupled to the input of the skew corrector (the uncorrected signal) and the other input of the MUX is coupled to the skew corrector output (the corrected signal). During normal operation (normal mode), the MUX selects the skew corrector output to provide a skew corrected signal. During testing (test mode), the MUX selects the uncorrected signal to eliminate the effects of the skew corrector, thereby bypassing the skew corrector.
For a MUX to provide the desired bypass function during testing, a MUX control signal must be set to an appropriate value. For example, a test program for an ATE may include a portion that activates the bypass function by setting a bit in a control register in the IC that defines the MUX control signal. However, including program steps to control a bypass MUX may undesirably increase the time required for testing preventing the full benefit of a bypass MUX from being realized.